1. Field of the Invention
The present invention relates to a sputtering method of continuously forming a film on a belt-like substrate while the substrate is moved, and to a method of producing a photovoltaic device using the sputtering method.
2. Related Background Art
In the magnetron sputtering technique, a plasma focusing magnet generates a magnetic flux in the form of a tunnel, and electrons generated by ionization as well as secondary electrons generated by sputtering are captured by the tunnel-like magnetic flux in the vicinity of the surface of a target. As a result, there are a high density of electrons near the surface of the target. This brings about a high probability of collision between the electrons and neutral gas molecules, thereby increasing the plasma density. Therefore, the magnetron sputtering technique has advantages that the speed of forming a film by sputtering is high and that a substrate on which a film is formed is not damaged by bombardment of high-energy secondary electrons because the electrons are confined in the vicinity of the target surface by the magnetic field. However, the density of plasma increases on a local portion of the target surface, whereby erosion of the local portion is enlarged by sputtering, whereby the target is not used uniformly. Therefore, the magnetron sputtering technique has a problem that the target is used with a low efficiency.
In order to expand the region of the target subjected to sputtering to improve the use efficiency of the target, the following techniques are known. (1) The magnet assembly for generating the magnetic field is mechanically moved. (2) The location of the plasma is moved by controlling the distribution of effective magnetic flux density using a combination of magnets. (3) The magnet assembly is modified to improve the distribution of magnetic flux in the region near the target, thereby expanding the plasma region.
Japanese Patent Application Publication No. 3-51788 discloses that a plasma focusing magnet is disposed at the back side of a target, a flux guide typically made of permalloy is disposed between the target and the magnet, and the flux guide is rocked to move the concentrated region of the magnetic flux.
FIG. 8 is a cross-sectional view for showing an example of a magnetron sputtering apparatus including a magnet assembly adapted to be mechanically moved. In FIG. 8, a target 1 is disposed on a target cooling plate 2 such that they are in intimate contact with each other. At the back side of the cooling plate 2 (i.e., the side opposite to the side of locating the target), there is disposed a magnet assembly 6 including permanent magnets 3 and 4 and a permanent magnet supporting member 5. The adjacent magnets 3 and 4 disposed on the side of the cooling plate 2 have magnetic poles opposite to each other such that lines of magnetic force emerging from the magnet assembly 6 returns to the magnet assembly 6 after passing through the surface region of the target 1, that is, a closed loop shape of tunnel-like magnetic flux is formed as shown by broken lines in FIG. 8. To increase the utilization efficiency of the target 1, the tunnel-like magnetic flux is rocked by moving the magnet assembly 6 along a circular path (means for moving the magnet is not shown in FIG. 8). A negative DC voltage or a high frequency voltage is applied to the target 1 (from a power supply not shown in FIG. 8) to generate a plasma, thereby sputtering the target.
As the method of successively forming functional deposited films of a photovoltaic device on a belt-like substrate, there is proposed a method of arranging a plurality of deposition chambers independent from one another and forming respective semiconductor layers in the respective deposition chambers. U.S. Pat. No. 4,400,409 discloses a continuous plasma CVD apparatus employing a roll-to-roll system. In this apparatus, a plurality of glow discharging regions are arranged. A flexible substrate with a sufficiently large length and a desired width is continuously moved in a longitudinal direction such that it is passed through the plurality of glow discharging regions from one region to another and depositing respective semiconductor layers of desired electroconductive types when it passes through the respective glow discharging regions. This technique makes it possible to continuously produce devices having semiconductor junctions. In the patent cited above, dopant gases used in each step of the semiconductor layer production are isolated from one another by gas gates so as to prevent the dopant gases from diffusing from one region into another thus preventing the glow discharging regions from being contaminated. More specifically, the glow discharging regions are isolated from each other by slit-like isolation paths through which isolation gas such as Ar or H.sub.2 is passed.
However, when such a roll-to-roll system is implemented with magnetron sputtering apparatus of the type described above, the speed of the tunnel-like magnetic flux varies as a sine wave in the direction of moving the belt-like substrate because the magnet assemblies are moved along circular paths while the belt-like substrate is moved at a constant speed. In other words, the speed of the tunnel-like magnetic flux has alternately positive and negative values. As a result, the sputtering time varies depending on the location of the plate-like substrate. This bring about nonuniformity of the film thickness along the direction of moving belt-like substrate.